1. Field of the Invention
The present invention relates to a laminated heat exchanger constituted by laminating tube elements and fins alternately over a plurality of levels and used for a cooling cycle and the like in air conditioning systems for vehicles. In particular, the present invention relates to a laminated heat exchanger that employs a structure in which a pair of tank portions are formed at one side of each tube element.
2. Description of the Related Art
In a laminated heat exchanger of this type, as disclosed in Japanese Unexamined Patent Publication No. H4-32697, tube elements are laminated alternately with fins over a plurality of levels. A pair of tank portions are formed at one end of each tube element with the pair of tank portions communicating with each other through a U-shaped passage portion. Adjacent tube elements communicate as necessary through the bonding of their tank portions, a plurality (three, for instance) of shoal-like beads are formed in the area of each tube element where the tank portions change into the U-shaped passage portion and the shoal-like beads that face opposite are flush to each other and bonded.
However, when a rupture test is performed on a laminated heat exchanger structured as described above by pumping high pressure fluid (at 30-40kg/mm.sup.2) into the tank portions, the bond between the shoal-like beads is broken in the tube elements located near the two ends in the direction of lamination. It has become clear that, as a specific phenomenon among the shoal-like beads, the rupture occurs starting with the bead at the central area (bead 26b in the example shown in FIG. 4). This is due to larger deformation occurring in the central area than at the ends of the tank portions as the number of laminated tube elements increases.
Such a rupture test conducted on a heat exchanger in which a special communicating passage, extending in the direction of lamination, is provided between the tank portions to induce the heat exchanging medium to specific tank portions via the communicating passage (a heat exchanger such as that shown in FIG. 9), has shown that the rupture occurs starting with the shoal-like bead 36c located near the connecting portion where the communicating passage is connected. The cause of the rupture is that the tank wall portion that faces opposite the connecting portion of the communicating passage becomes distended by the pressure of the fluid coming through the communicating passage, as indicated with the broken line, making the quantity of deformation larger than in the other areas of the tank portion.